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Is this only for vertical lines or for others too? Could you attach a small testdataset to reproduce the problem?

Seems to be vertical lines only.
I attach a shapefile. Turn on arrows decoration on symbology-ng and orientation symbol on labels. Arrow decorations are correct.

In particular, edges with id 5 and 7 of qgisbug3543_1.zip are disagreed upon between label and line decoration. They are both going down, so line decoration is correct and label orientation symbol is wrong.

I'd add that the original testcase only contained lines oriented from bottom up, which may explain why this case wasn't cought.

The attached patch (bug3643.diff) fixes it for me, for both datasets.
Might not be extremely elegant, but should be also more robust..

A slightly more elegant version:

diff --git a/src/core/pal/feature.cpp b/src/core/pal/feature.cpp
index 6f657b1..5501434 100644
--- a/src/core/pal/feature.cpp
+++ b/src/core/pal/feature.cpp
@@ -596,7 +596,7 @@ namespace pal
       {
         //std::cout << alpha*180/M_PI << std::endl;
         if ( flags & FLAG_MAP_ORIENTATION )
-          reversed = ( alpha > M_PI / 2 || alpha < -M_PI / 2 );
+          reversed = ( (ex == bx && ey < by ) || ( alpha > M_PI / 2 || alpha < -M_PI / 2 ) );

         if (( !reversed && ( flags & FLAG_ABOVE_LINE ) ) || ( reversed && ( flags & FLAG_BELOW_LINE ) ) )
           positions->push_back( new [[LabelPosition]]( i, bx + cos( beta ) *distlabel , by + sin( beta ) *distlabel, xrm, yrm, alpha, cost, this, reversed ) ); // Line

Even smaller is just moving re-adding the equal sign but to the right part:

-          reversed = ( alpha > M_PI / 2 || alpha < -M_PI / 2 );
+          reversed = ( alpha > M_PI / 2 || alpha <= -M_PI / 2 );

Works fine in my case, altought the equality comparison between the return from atan2 and M_PI/2 looks scary (miracles!)

This final version seems to be the closest to the original code, where the equal sign was in the first rather than second side of ||.

Great. I confirm the fix. You can close the grave now :)

How do we reopen this bug ? I personally can't change the "Status" field.

I don't really get the meaning of "Map" vs. "Line" orientation, btw

The following changes since commit 59be561a4d9100fb2084006c2acc7df5798c52fc:
Juergen E. Fischer (1):
fix warnings and more cosmetics

are available in the git repository at:

[email protected]:strk/Quantum-GIS.git dirsym

Sandro Santilli (1):
Label direction symbol shouldn't depend on "map" vs. "line" orientation. Refer to issue #3643 for further discussion

src/core/pal/feature.cpp |    5 +---
 1 files changed, 2 insertions(), 3 deletions(-)

My "dirsym" branch implements solution 2 of Mayeul (as per this ticket subject).
For solution 1 see bug #4424

I'm not seeing that from the logs ?
Unless the commit log is broken:
https://issues.qgis.org/projects/quantum-gis/repository/revisions/697b35a4185df804baa0f648389a05ed8a7d59fa